Many airborne vehicles require accurate, but low-cost, self-contained navigation systems. Doppler radars can provide accurate velocity data but the total navigation solution requires accurate heading a well. Magnetic compasses can provide this heading, but currently available units are large, heavy and costly. A fundamental aspect of any magnetic heading sensor is the need to measure heading only in the horizontal plane. Any contamination of this measurement introduced by a portion of the vertical magnetic field of the earth, caused by a tilt (pitch or roll) error, results in a large heading error. FIG. 1 shows the sensitivity of magnetic heading accuracy to tilt error, as a function of magnetic dip angle. The dip angle is the arctangent of the ratio of the earth's vertical to horizontal magnetic field components.
Previous magnetic compasses have used a number of techniques to derive heading in an airborne vehicle. One technique is to use a magnetic headng sensor that is pendulously suspended in a fluid. The effect of the pendulum is to cause the magnetic sensing element to remain horizontal during un-accelerated flight, so that heading is properly measured. Vehicle accelerations, however, cause the pendulum to depart from the vertical, resulting in significant heading errors from the magnetic sensors. Another approach is to use a body-mounted triad of magnetic sensors and transform their outputs into the horizontal plane using pitch and roll from a separate vertical sensor or vertical gyro. A typical vertical gyro is algined or slaved to the local vertical using simple accelerometers or even liquid levels. This technique results in satisfactory pitch and roll accuracy during un-accelerated flight but is degraded during maneuvers.
Accurate pitch and roll can be provided by an inertial system, i.e., a combination of accelerometers and gyroscopes that are arranged so as to operate as two Schuler-tuned loops. These loops, when properly initialized prior to take-off, retain their knowledge of pitch and roll even during severe maneuvers. The major disadvantages of such an inertial system are its high cost and complexity.